It is well known to inject miscible gases into a subterranean hydrocarbon-bearing reservoir to improve production of hydrocarbons from the formation through miscible flooding. Miscible flooding occurs when a gas injected into the formation mixes with hydrocarbons in the formation at the ambient temperature and pressure of the formation to displace the hydrocarbons. This phenomenon occurs as a result of intermediate hydrocarbon compounds in the injected gas condensing upon contact with the formation hydrocarbons and altering the composition of the formation hydrocarbons. If sufficient amounts of hydrocarbon intermediates condense from the gas and mix with the formation hydrocarbons, the flooding gas and formation hydrocarbons become miscible. This phenomenon is referred to as the condensing mechanism. The injected gas may also become miscible with the formation hydrocarbons through the vaporizing mechanism in which intermediate hydrocarbons are vaporized from formation hydrocarbons into the injected gas, thus creating miscibility. Flooding a subterranean formation with a gas which is miscible with hydrocarbons present in the formation versus a gas which is immiscible with these hydrocarbons can result in recovery of up to about 10-15 volume, % of incremental hydrocarbons in place in the formation.
A process for miscibly flooding a subterranean formation utilizing a gas, such as carbon dioxide, methane, or nitrogen, which does not contain intermediate hydrocarbons and which is capable of extracting or vaporizing formation hydrocarbons so that the gas becomes enriched with extracted formation hydrocarbons is conducted by raising the operating pressure of the flooding process so as to reach the minimum miscibility pressure, that is, the minimum pressure at which the gas is miscible in the formation hydrocarbons. The process of miscible flooding can also be conducted by enriching the gas with hydrocarbon intermediates prior to injecting the gas into the subterranean formation. The resulting enriched gas is a multicomponent gas containing sufficient hydrocarbon intermediates to render the enriched gas substantially miscible in formation hydrocarbons upon injection into the formation.
An example of one type of enrichment process is found in U.S. Pat. No. 4,529,037 which discloses the formation of a drive gas comprised of carbon dioxide and intermediate hydrocarbons. A surface facility is employed to maintain multiple phase mixtures of crude oil and carbon dioxide in an extraction zone from which it is withdrawn for subsequent injection into a reservoir. The cost of the necessary facilities to extract hydrocarbons and mix the extracted hydrocarbons with a drive gas can, however, be prohibitively expensive for some reservoirs.
In order to reduce the cost of drive gas preparation it has been suggested to utilize the mixing that takes place in the reservoir to create an enriched gas. U.S. Pat. No. 2,875,832, for example, discloses a process which involves introducing carbon dioxide into the oil reservoir and recycling the gaseous effluent back into the formation. Because the process requires the injection of carbon dioxide and mixtures of carbon dioxide and effluent gases to be carried out at elevated pressures, i.e. at least about 300 psi, the overall process is still too expensive for many reservoirs.
In many instances, the minimum miscibility pressure of a subterranean hydrocarbon-bearing formation may exceed the fracture pressure of the formation thereby rendering miscible flooding impractical. At offshore locations where space is at a premium and gas plants are nonexistent, operators may be forced to reinject gas into a producing subterranean formation. Thus, a need exists for a process for preparing miscible gases for injection into a subterranean hydrocarbon-bearing formation.
It is therefore a broad object of the present invention to reduce the cost of a miscible flooding process, and a more specific object to reduce the cost of a gas enrichment process.
It is a further object of the present invention to provide a process for enriching a gas for miscible flooding where surface facilities must be minimized due to constraints on available space.